This invention relates to the field of cathode ray tubes (CRT) for monitors, TV, etc., and particularly to the prevention of radiation exposure for the CRT monitor operator through elimination of the causative electric fields.
As is well known, all electrical and electronic devices employing time-varying, high voltages emit electric fields unless the fields are cancelled or eliminated in some fashion. In CRT's as used in display monitors and television sets, a measurable field can be generated by the flyback voltage pulses from the horizontal deflection circuitry being coupled to the inner coating of the CRT. With the growing use of CRT monitors in many business and recreation activities, concern has also grown about possible danger to the operator from emitted electric fields although little is known about the actual effects of such fields. Some countries have already set up requirements as to the allowable amount of radiated field in the area in front of and surrounding the CRT monitor; other countries are in the process of doing so. Therefore, manufacturers of monitors are attempting to achieve acceptable radiation levels according to known measuring protocols.
The most common way of reducing radiation in the area adjacent to the operator has been to provide a conductive, grounded panel on the faceplate of the monitor. Such panels usually include an extremely thin coating of indium-tin oxide, applied in a vacuum by a very critical processing technique. The coating is applied to a separate, curved precision panel of glass which is then resin-bonded to the outer surface of the CRT faceplate. While this method is generally satisfactory, it is relatively expensive to produce and does have an undesirable effect on monitor quality.
A different approach has been described in the literature which attempts to "cancel" any electric field in front of a CRT. In this method the unwanted fields are partially cancelled by, for example, providing radiators on the front of the CRT, adjacent the picture area. A pickup or sensor is provided to sense any field signal in that area, amplifying and inverting the sensed signal and coupling it to the radiators to produce an opposing field. Other positions of the radiators are discussed. It is suggested that, while the requirements of a specific measurement protocol may be met by a particular radiator design, there may also be relatively large lobes of radiated signal in areas between the required points of measurement. In summary, it is apparent that, while this type of cancellation method is less expensive than previous shielding methods, it leaves much to be desired since creating a "cancellation" field cannot provide overall elimination of the unwanted radiation as is needed. A better approach is therefore still needed.